Engineering Advanced Process control (APC) uses measurements of important process variables to incorporate into a feedback loop for control. The process input parameters are adjusted based on the measurements of the process output products. The process parameter set points may be varied to implement batch control for wafers.
Critical dimension (CD) is a key process parameter in photolithography, and a great deal of effort has been directed towards CD control. However, CD control is more complex than merely using feedback between the stepper and the CD metrology tool, because there is an etch bias between the after develop inspection (ADI) dimension and the after etch inspection (AEI) dimension. The etch bias may result from photoresist shrinkage or deviation from the scanning electron microscope (SEM) measurement of the photoresist CD. The etch bias is a significant problem, especially for 90 nanometer technology and more advanced technologies. In conventional systems, ex-situ CD SEM data based on the ADI measurement of the CD are fed forward to change the etch recipe (i.e., the etching parameters, such as etching time). The goal is to adjust the etch recipe to meet the target AEI CD. U.S. Pat. No. 6,350,390 is incorporated by reference herein in its entirety, as though set forth fully herein. U.S. Pat. No. 6,350,390 describes a method for correlating the ADI and AEI critical dimensions.
Unfortunately, the ADI CD and AEI CD do not have a very strong correlation. The ADI CD measurement (of the photoresist) is unstable and unreliable, due to shrinkage of photoresist, beginning with 130 nm technology, and especially when 90 nm technology and more advanced technologies are used. Frequently, feed forward adjustments to the etch recipe based on the ADI CD do not produce the target AEI CD with the desired accuracy. The measured CD value depends strongly on the shape of the sidewall of the measured structures. For example, predictions of the AEI CD based on the ADI CD are likely to be inaccurate if the shapes of the sidewalls of the measured structures are different from the shapes of the sidewalls of the standard samples used to calibrate the relationship between the AEI CD and the ADI CD. Precise control using ADI feedforward information requires photoresist profile information and a clear understanding of the relationship between the feature profile, CD and etching behavior. This is very difficult to obtain.